Fused pyrosulfate-halide solvent electrolyte



Patented Nov. 8, 1949 FUSED PYROSULFATE-HALIDE SOLVENT ELECTROLYTE Charles E. Berr, Hartford, Conn., assignor to Wesleyan University, a corporation of Connecticut No Drawing. Application April 27, 1944, Serial No. 533,048

2 Claims. (01. 204-64) This invention relates to the production of metals and alloysby electrolytic reduction of metal compounds, and refers more specifically to electrolytic reduction conducted in molten salt.

Many electrolytic reduction methods involve the use of molten salt, for instance a halide or oxyhalide, as a solvent for a metal compound to be reduced. Most frequently, chlorides or fluorides are used, for instance sodium or potassium fluoride or chloride, or their mixtures, or cryolite.

Although halides melt at relatively lower temperatures than many other common salts, this being one of theadvantages of halide solvents, on occasion it would be desirable to lower still further the melting point of the bath by adding or substituting a suitable other salt. Such other salt should not decompose during electrolysis, nor react with nor contaminate the metal or alloy product of the electrolytic reduction process.

I have discovered that the pyrosulfate of an alkali metal, preferably potassium pyrosulfate (K2S2O7), is a practical and effective solvent, either alone or in admixture with metal halides, including for instance cryolite (NaaAlFs) and simpler alkali metal halides. The pyrosulfate or mixture has a lower melting point than most simple halides, and is well adapted for use as a solvent for fused-salt electrolytic reductions of metal compounds. The pyrosulfate and mixture are stable and do not react with, or materially contaminate, the metal product of electrolysis, even when such product is a readily oxidizable metal such as for instance aluminum, beryllium, or rare earth metal. This result is the more surprising in view of the general experience in the art that salts of oxyacids are generally too reactive towards molten, readily oxidized metals to be used in fused salt electrolytes.

The invention is a fused solvent electrolyte for use in an electrolytic method which comprises electrolyzing a molten anhydrous solution of reducible metal compound, for instance a rare earth or a halide of a rare earth metal or beryllium.

More particularly, the invention is a fused solvent electrolyte consisting of a salt mixture, for use as a molten solvent of metal compounds in the electrolytic reduction of such compounds, the mixture comprising essentially at least one alkali metal halide having a melting point above 400 0., which halide is in major proportion, and l a minor proportion of a pyrosulfate of an alkali metal. Preferably, the halide is potassium or sodium chloride or fluoride. The relative proportions of halide and pyrosulfate are not critical, but may be varied considerably. The optimum proportions will depend, of course, on the particular halide or mixture of halides used and on the degree of lowering of melting point desired. A proportion between 3% to 15% by weight of potassium pyrosulfate is suggested as generally satisfactory.

The metal compound to be reduced may be any which dissolves in the pyrosulfate or the pyrosulfate-halide mixture, for instance a metal halide or oxyhalide, or a metal oxide. This invention is not limited to any particular compounds, but relates to the composition of the solvent for such compounds.

A preferred solvent according to this invention comprises cryolite (NaaAlFa) and potassium pyrosulfate, the former in major proportion and the latter in minor proportion, for instance 5 to 10% by weight. In practice, the relative proportions of sodium and aluminum fluorides may depart to some extent from that indicated by the formula NaaAlFs, and the sodium may be partially or wholly replaced by potassium.

It is entirely feasible to form the pyrosulfate in situ by heating the corresponding bisulfate above 400 in admixture with the other ingredient or ingredients of the solvent mixture.

The solvent mixture may also include other ingredients customarily used in fused salt electrolytes, for instance alkaline earth metal chlorides, borax or boron oxides, in minor amounts.

I claim:

1. A fused solvent electrolyte consis ing substantially of at least one alkali metal chloride and at least one alkali metal pyrosulfate, said alkali metal pyrosulfate being present in an amount between 3% and 15% by weight of said electrolyte, said electrolyte having the characteristic of being a substantially inert solvent for rare earths, halides of rare earth metals, and beryllium halides.

2. A fused solvent electrolyte consisting substantially of at least one alkali metal chloride and potassium pyrosulfate, said potassium pyrosulfate being present in an amount between 3% and 15% by weight of said electrolyte, said elec- 3 trolyte having the characteristic of being a substantially inert solvent for rare earths, halides of rare earth metals, and beryllium halides. gg CHARLES E. BERR. 6:751 5 5,132

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 10 Number Name Date 1,118,209 Mattes Nov. 24, 1914 1,564,139 Saklatwalla Dec. 1, 1925 1,905,866 Heavy Apr. 25, 1933 2,231,030

Scofleld et a1. Feb. 11, 1941 15 FOREIGN PATENTS Country Date Germany Mar. 28, 1929 Great Britain Of 1893 Norway Jan. 4, 1897 OTHER REFERENCES 

